My Science School

Volcanic bombs

A major volcanic eruption can hurl boulders high into the air. These boulders, called volcanic bombs, can be very large. Most of the material thrown out of the erupting volcano is ash, which forms a huge cloud. Steam and sulphurous gases are also released, and these can be very dangerous to bystanders. Pumice is a kind of foamed rock that is often thrown out during an eruption. It is full of tiny holes, making it very light. It is formed when gases in molten rock are boiled off; creating foam that solidifies as it cools. 

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What is the Ring of Fire?

All around the rim of the Pacific Ocean is a circle of places of volcanic activity known as the Ring of Fire. The Ring of Fire is caused by shifts in the giant plates that make up the Earth’s crust. The Pacific plate is gradually disappearing under the surrounding continental plates, and volcanic activity marks the points where this movement is taking place. Frequent earthquakes accompany the volcanic action in the countries surrounding the Pacific. Also, ocean trenches and underwater volcanoes form around the Ring of Fire. 

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What is lava?

Lava is molten rock that reaches the Earth’s surface. Its temperature may be as high as 1, 200 degree Celsius. Lava may be as runny as water or so thick that it scarcely moves at all. The liquid type of lava that flows from Hawaiian-type volcanoes runs downhill in sheets and streams, travelling very fast. Sometimes the surface of these streams cools and hardens, and the liquid lava continues to flow underneath, eventually producing lava tubes when it cools. The thicker type of lava inches its way down the side of a volcano like a very slow avalanche, carrying with it anything in its path, as well as red-hot lava. 

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What happened to Pompeii and Herculaneum?

Two busy Roman towns were completely buried in lava and ash by an eruption of Mount Vesuvius in AD 79.

A burning rain of ash, lava and volcanic debris covered Pompeii to a depth of more than 3 m. Nearby Herculaneum was buried under about 18 m of ash. Many people were overcome while trying to escape the eruption. In Pompeii, cavities were found in the ash where their bodies had lain and then rotted away. The two towns preserved everyday Roman life, and they are still being excavated. 

 

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What is a volcano?

Volcanoes are openings in the Earth’s crust through which molten lava, red-hot rocks, steam and fumes pour out. We usually think of a volcano as a smoking mountain that erupts explosively, but most volcanoes are simply cracks in the crust through which lava flows continuously.

The more familiar kind of volcano has a single central pipe though which lava reaches the surface, building up into a cone as it cools. The cone consists of layers of lava and volcanic ash. There is usually a crater at the centre. Most active volcanoes smoke and spit out occasional pieces of lava, and dramatic eruptions are rare.

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How fast do volcanoes grow?

Unlike most geological happenings, the growth of a volcano can be very rapid. In 1943, a farmer in Mexico noticed smoke coming from a crack in the ground in his cornfield. Lava began to ooze out, and six days later it had piled up into a volcanic cone 150 m high. By the end of the year the cone had grown to 450 m. The eruption finally stopped in 1952, when the volcano Paricutin had reached a height of 2,808 m above sea level. The volcano had buried two whole villages. 

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How can rocks be folded up?

Most rocks are too brittle to be reshaped, but folding is common in sedimentary rocks. Movements in the Earth’s crust force flexible sedimentary rocks into folds. These folds are sometimes so large that the rock lays back on itself in layers. Geologists may find that the layers of rock are actually upside down, with the youngest rocks on top. 

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What is a mountain range?

A group of mountains is called a range. Nearly all large mountains are grouped in ranges. Single high peaks are usually old volcanoes. Mount Kilimanjaro in Tanzania is the highest mountain in Africa. It is a single peak, and is an extinct volcano. These isolated peaks always look much higher than mountains in ranges, because they are not surrounded by the usual lower foothills that disguise their true height.

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Why do clouds form near mountains?

Clouds form when damp air is forced upwards to pass over a mountain range. Air is forced up the face of the mountain because it is unable to go around it. There is usually a high wind on the top of the mountain. As it rises, the air becomes cooler and the moisture in the air condenses to form water droplets, resulting in the formation of clouds. This explains why there is often high rainfall and snow in mountainous regions.

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What is a rockslide?

Rock is broken away from the mountainside by the effects of freezing. Rainwater enters cracks in the rock and as it freezes, it expands and opens the crack wider. As the thaw begins the pieces of rock splinter and separate from the bedrock. Loose rock builds up continuously on a mountainside, but usually only small pieces slide down. This loose broken rock is called scree.

A rockslide takes place when the mass of broken pieces of rock slides down the side of a mountain. This usually happens in very wet weather, when rain lubricates the rock and allows it to slide freely down the mountainside, mixed with a torrent of mud.

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Are climbers able to breathe at high altitudes?

The higher you climb, the thinner the air becomes, until there is not enough oxygen to support life. On the highest mountains, there is just about enough oxygen for life to survive for a short time. Extremely fit mountaineers have managed to climb to the top of Mount Everest without the use of oxygen cylinders.

Our bodies can adapt slowly to a lack of oxygen, which is why expeditions to the Himalayas and other high ranges camp at progressively higher levels to get used to the thinner air. People living at high altitudes in the Andes and the Himalayas develop enlarged chests and changes in their lungs to help them breathe.

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What is chalk?

Chalk is formed from the skeletons of millions and millions of tiny animals called foraminifera. It is a sedimentary rock that formed millions of years ago beneath shallow seas. Chalk is used in the manufacture of rubber goods and paint. 

 

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